One proposed hybrid vehicle is equipped with an air conditioning system to warm up a passenger compartment with an engine as a heat source and intermittently operates the engine, based on a measured internal temperature of the passenger compartment, a target temperature set in the air conditioning system, and a cooling water temperature of the engine (see, for example, Japanese Patent Laid-Open Gazette No. 9-233601). This prior art hybrid vehicle drives the engine to ensure the sufficient level of heating performance even at a stop of the vehicle or in a motor drive mode using an electric motor alone for driving, when a temperature difference ΔT between the target temperature set in the air conditioning system and the measured internal temperature of the passenger compartment is not less than a preset reference level, which depends upon the cooling water temperature of the engine.